Adverse events following immunisation (AEFI) in the National Vaccination Programme of the Netherlands (RVP) have been monitored through an enhanced passive surveillance system by RIVM since 1962. From 1984 onwards evaluation is done in close collaboration with National Health Council. Reports from Health Care workers are recieved mainly by telephone through the operating vaccine information and advisory service. Further data are obtained, if necessary, from parents, general practitioners, peadiatricians etc. After supplementation and verification of data a (working) diagnosis is made and causality assessed. In this report on 1996 and 1997 an overview of all received AEFI is presented with classification according to case definitions and causality. Reporting bias, background rates of specific events and possible pathophysiology of symptoms are discussed. On a total of approximately 2 million vaccinations per year 732 and 822 AEFI were submitted in 1996 and 1997 respectively. Of these 1,6% (12) and 2% (17) were unclassifiable because of missing information. In 78% (565) and 80% (642) of the 22% (155) and 20% (163) the events were judged to be coincidental. Compared to 1995 the number of reports stabilised, with similar distribution and causality.